Preservation of fibrous cellulosic material



Patented June 11, 1940 UNITED STATES PATENT OFFICE Ernest It. Boiler, Cleveland Heights, Ohio, as-

signor to E. I. du Pont de Nemours 8; Company, Wilmington, DeL, a corporation of Delaware No Drawing. Application June 22, 1937, Serial No. 149,897

This invention relates to the treatment of cellulosic fibers as in the manufacture of felted products for the preservation of such products from attack by fungi and insects, and more particularly to processes wherein a water-soluble preservative is fixed to the fibers by heating a suspension of cellulosic fibers in an aqueous solution containing zinc and chromate ions.

Various procedures have been suggested for treating wallboard, paper and similar felted cellulosic products with toxic substances for the prevention of deterioration and decay due to attack by fungi and insects. The processes most desirable for this purpose are those wherein the toxic materials are incorporated in the product during the process of manufacture.

Processes of this type heretofore available, however, have employed water-insoluble compounds which were either dispersed in the pulp slurry as such or formed therein by precipitation. Waterinsoluble compounds, however, are generally accepted as having rather low fungicidal efiiciency. Moreover, they are more or less mechanically held among the fibers and consequently do not offer uniform protection to the individual fibers.

In the preservatiton of wood it has long been recognized that preservatives having an appreciable solubility in water are highly efiective against the penetration of fungal attack. These preservatives, however, have found little application in the treatment of felted cellulosic products, particularly in the wet end of a felting process, because the large volumes of water used in such processes tend to hold the soluble preservatives in solution rather than deposit them on the fibers.

chromium to be sorbed or otherwise fixed to the fibers.

Prpcesses according to this invention may be carried out advantageously at any suitable point in the wet end of a felting process, and in many cases may be adapted to existing felting processes without change in equipment or mode of operation. For example, in many of the methods used for refining fibrous raw materials for the production of felted cellulosic products, such as wallboard, pulp slurry, in the course of normal operation, is subjected to sufllciently high temperatures to cause practically complete sorption of the zinc and chromium. In other cases the processes of this invention may be adapted to existing processes and apparatus with little change in operating practice.

No special technique is required for adapting the process of this invention to existing felting processes, as apparatus for heating pulp slurries are well known to those skilled in the art and are either available in existing felting processes or may be installed with little change in existing plant layouts. All that is required is the forma- I tion of a pulp slurry containing zinc and chromate ions, and the provision of means for heating the a slurry at a temperature and for a length of time sumcient to cause a substantial portion of the zinc and chromium to be fixed to the pulp fibers. Zinc chloride and sodium dichromate may be used advantageously as a source of zinc and chromate ions.

In standard felting processes the fiber suspension is frequently very dilute, often amounting to not more than two per cent dry fiber. Notwithstanding this high degree of dilution, the processes of this invention have been found to be highly eflicient in that a minor amount only of zinc and chromium remain in the white water after the felting operation, the major amount being taken up by the fiber.

In the usual felting processes used in the manufacture of wallboard and similar felted cellulosic materials it is general practice to pass the wet felted mat thru a drying oven. Usually the temperature in the oven will vary from 350-450" F. I have found that material treated according to the processes of this invention will withstand these drying temperatures without charring or burning. In this respect the treated material compares favorably with the untreated. This is quite surprising, in view of the fact that similar material containing zinc chloride and sodium dichromate in solution is badly charred under the same conditions.

Moreover, when felted material containing unaltered zinc chloride alone or in combination with sodium dichromate is heated to temperatures encountered in the drying oven, a certain amount of hydrochloric acid is evolved. This leads to a severe corrosion problem. When, however, the zinc and chromium are fixed to the fibers by processes according to this invention, corrosion of the drying equipment is avoided.

The mechanism by which zinc and chromium are removed from the solution is not definitely understood, altho it is entirely possible that the zinc and chromium are fixed in the fibers by selective sorption or by the formation of a difficultly soluble complex with organic constituents of the fibers. In any event, it has been found that neither zinc chloride nor sodium dichromate alone is removed from solution to a satisfactory extent by the same procedure.

Whatever the mechanism of the reaction which causes fixation of the zinc and chromium, whether it be selective adsorption, the formation of a complex among the zinc, chromium, and organic constituents of the fibers or by some other phenomenon, the zinc, chromium, and cellulosic material essentially contribute to the fixation of the preservative of the fibers in such a manner that substantially all the zinc and chromium is removed from the solution and fixed to the fibers in such a manner .that the material does not char or burn or evolve hydrochloric acid durin drying. This mutuality is evident from. the fact that like results are not obtained with zinc chloride or sodium dichromate alone.

I have also found that certain materials, such as aluminum sulfate, accelerate the rate and the extent to which the zinc and chromium is removed from solution. This effect, tho small, is advantageous in that many of the customary felting processes employ aluminum sulfate to adjust the pH of the pulp slurry.

In order that the invention may be more fully understood reference may be had to the following examples, in which the parts are by weight unless otherwise specified:

Eazample I According to this example a zinc chloride-sodium dichromate mixture containing equal parts of zinc chloride (ZnClz) and sodium dichromate dihydrate (NazCr2O-L2H2O) was added to a pulp slurry containing 4 per cent dry pulp acidulated with 0.02 per cent aluminum sulfate. The zinc chloride-sodium dichromate preservative was used in concentration of 2 per cent total salt based on the dry weight of the pulp. The so treated fiber suspension was then heated to 200-210 F. and maintained at that temperature for two hours, after which the pulp was felted in the usual manner. Analysis of the white water indicated that substantially all of the zinc and about 95 per cent of the chromium had been removed from solution.

The resulting product was found to resist the attack of the fungi, Lenzites trabea, Fomes annosus, and Aspergillus niger, under conditions in which the growth of the same fungi develop very rapidly thruout untreated controls.

Example II In this example the concentration of zinc chloride-sodium dichromate was increased to twice that of Example I and the time of digestion was extended to three hours. Otherwise the procedure was the same as Example I. Analysis of the white water indicated that substantially all of the sodium dichromate and about 75 per cent of I the zinc chloride had been removed from the solution. The felted and dried material was also subjected to a leaching test which showed less than one per cent leaching of zinc and no detectable leaching of chromate.

Example III According to this example 0.8 per cent based on the dry weight of the pulp of zinc chloridesodium dichromate containing four parts of zinc chloride and one part of sodium dichromate was added to a three per cent pulp slurry, which had previously been adjusted to a pH of 4.9 by the addition of aluminum sulfate. The treated slurry was then held at a temperature of 200 F. for six hours without beating. At the end of this time all the sodium dichromate and 67.5 per cent of zinc chloride had been removed from the solution.

Altho the finished material of this example contained a much smaller quantity of preservative than that of Example I, it was found to completely inhibit the attack of Lenzztes trabea. A slow growth of Fomes annosus and Aspergillus niger was observed. The growth of these fungi, however, was materially inhibited as compared with the growth of the same fungi in untreated controls.

In using the processes according to this invention the amount of zinc chloride-sodium dichromate preservative required depends upon the concentration of the pulp slurry and the amount of preservative desired in the final product.

Thus, for a pulp slurry containing about four per cent dry fibers, at least about 0.08 per cent of the zinc chloride-sodium dichromate preservative would be required to produce a finished board having approximately two per cent preservative content. If desired, the amount of zinc chloridesodium dichromate mixture added may be increased to allow for the amount carried away in the white water. This, of course, would be unnecessary in plants where the white water is recirculated. Other factors such as the selective sorption of the zinc and chromate ions may be considered in determining the amount of preservative required. I

The extent of the sorption and the rate at which it takes place is largely a function of the temperature and time, but other factors such as the nature of the pulp being treated are also involved. In general, I have found temperatures above 150' F. the most suitable for the ordinary run of pulp used in fabricating wall-board and like felted cellulosic products. At or near boiling temperatures, two hours will ordinarily be found ample, altho good results may be obtained with shorter periods. With low temperature, however, it has been found desirable to continue heating for several hours.

Temperatures at or near boiling constitute a practical maximum where pressure equipment is not available. In many instances. however, fibrous raw material is digested under steam pressure to effect disintegration of the fiber. The preservative may be fixed at this point in the wet end of the felting process if desired as the subsequent treatments to which the pulp fibers are subjected do not materially affect the fixation of the preservative.

As already pointed out, the zinc chloride and sodium dichromate mutually contribute to the fixation of the preservative to the fibers as unsatisfactory fixation is obtained with either of the constituents alone. For this purpose the proportion of zinc chloride to sodium dichromate may be varied over a wide range. Ordinarily, however, it has been found desirable to maintain the proportion of zinc chloride to sodium dichromate within the range of 1:1 to 5:1, as more satisfactory and economical results are obtained.

Altho I have disclosed specifically the use of zinc chloride and sodium dichromate mixtures, it is to be understood that my invention is not limited to this specific combination of salts, as I have found the presence of cellulosic fiber, zinc ion and chromate ion to be the principal constituents responsible for the fixation of the zinc and chromium to the pulp fibers. Thus I may employ dichromates of other metals than sodium and zinc salts other than the chloride. It should be observed, however, that the presence of other ions which would cause the formation of insoluble zinc salts or insoluble chromates should be avoided.

The processes according to this invention may be used for the treatment of cellulosic fibers alone but are particularly applicable to the treatment of pulp in the manufacture of cellulosic products such as wall-board, insulating board, acoustical board, and insulating paper, and for such purposes are particularly advantageous in that though a water-soluble preservative is used the preservative is fixed to the fibers in such a manner that the treated material is resistant to leaching.

I claim:

1. In a process for preserving felted cellulosic products wherein a preservative is incorporated in a felted product in the wet end of a felting process, the steps comprising forming a pulp slurry containing zinc chloride and sodium dlchromate in an amount equivalent to at least about 0.8 per cent on the basis of the dry weight of the pulp, heating the slurry at a temperature of about 150 F. or more for a time sufilcient to cause a substantial portion of the zinc and chromium to be fixed to the fibers and forming a felted product therefrom.

2. In a process for preserving felted celluloslc products wherein a preservative is incorporated in a felted product in the wet end of a felting process, the steps comprising forming a pulp slurry containing zinc chloride and sodium dichromate in the proportion of 1:1 to 5:1 and in an amount equivalent to at least about 0.8 per cent on the basis of the dry weight of the pulp, heating the slurry at a temperature of about 150-212 F. for a time sufficient to cause a substantial portion of the zinc and chromium to be fixed to the fibers and forming a felted product therefrom.

ERNEST R. ROLLER. 

